A 38-year-old Japanese man was referred to our outpatient clinic for treatment of infertility. Semen analysis showed azoospermia. Chromosome analysis revealed a 47XXY karyotype, and non-mosaic Klinefelter's syndrome (KFS) was diagnosed. Upon physical examination, the patient's right testicular volume was 30 mL and the left testicular volume was 3 mL. Laboratory tests showed normal levels of lactate dehydrogenase, alpha-fetoprotein, and human chorionic gonadotropin beta-subunit. The plasma luteinizing hormone and follicle-stimulating hormone levels were increased to 15.7 mIU/mL and 45.9 mIU/mL, respectively. The plasma testosterone was decreased to 0.25 ng/mL. Magnetic resonance imaging showed a right testicular mass of low-signal intensity on the T1-weighted image and of high-signal intensity on the T2-weighted image. Therefore, the final diagnosis was KFS with a right testicular tumor. Thus, a right high orchiectomy was performed. Histological examination revealed an epidermoid cyst of the right testis. Epidermoid cysts in cases of KFS are rare. To our knowledge, only seven cases, including ours, have been reported in the literature.